Kernel: Stop hardcoding syscall in signal trampoline.

We now no longer hardcode the sigreturn syscall in
the signal trampoline. Because of the way inline asm inputs
work, I've had to enclose the trampoline in the function
signal_trampoline_dummy.

Kernel/Process.cpp

@@ -793,26 +793,29 @@ void Process::sys$exit(int status)
     ASSERT_NOT_REACHED();
 }
 
-// The trampoline preserves the current eax, pushes the signal code and
-// then calls the signal handler. We do this because, when interrupting a
-// blocking syscall, that syscall may return some special error code in eax;
-// This error code would likely be overwritten by the signal handler, so it's
-// neccessary to preserve it here.
-asm(
-    ".intel_syntax noprefix\n"
-    "asm_signal_trampoline:\n"
-    "push ebp\n"
-    "mov ebp, esp\n"
-    "push eax\n" // we have to store eax 'cause it might be the return value from a syscall
-    "sub esp, 4\n" // align the stack to 16 bytes
-    "mov eax, [ebp+12]\n" // push the signal code
-    "push eax\n"
-    "call [ebp+8]\n" // call the signal handler
-    "add esp, 8\n"
-    "mov eax, 0x2d\n" // FIXME: We shouldn't be hardcoding this.
-    "int 0x82\n" // sigreturn syscall
-    "asm_signal_trampoline_end:\n"
-    ".att_syntax");
+void signal_trampoline_dummy(void)
+{
+    // The trampoline preserves the current eax, pushes the signal code and
+    // then calls the signal handler. We do this because, when interrupting a
+    // blocking syscall, that syscall may return some special error code in eax;
+    // This error code would likely be overwritten by the signal handler, so it's
+    // neccessary to preserve it here.
+    asm(
+        ".intel_syntax noprefix\n"
+        "asm_signal_trampoline:\n"
+        "push ebp\n"
+        "mov ebp, esp\n"
+        "push eax\n"          // we have to store eax 'cause it might be the return value from a syscall
+        "sub esp, 4\n"        // align the stack to 16 bytes
+        "mov eax, [ebp+12]\n" // push the signal code
+        "push eax\n"
+        "call [ebp+8]\n" // call the signal handler
+        "add esp, 8\n"
+        "mov eax, %P0\n"
+        "int 0x82\n" // sigreturn syscall
+        "asm_signal_trampoline_end:\n"
+        ".att_syntax" ::"i"(Syscall::SC_sigreturn));
+}
 
 extern "C" void asm_signal_trampoline(void);
 extern "C" void asm_signal_trampoline_end(void);